Process for producing aluminum and silicon from aluminum silicon alloys

ABSTRACT

NACL-KCL-ALCL3   OR NACL-KCL-ALF3. MOLTEN ALUMINUM IS PRODUCED AT THE CATHODE, ANODE RESIDUE IS ACID LEACHED TO PRODUCE METALLURGICAL GRADE SILICON. ALUMINUM-SILICON ALLOY PRODUCED, FOR EXAMPLE, BY SMELTING ALUMINUM-SILICATE ORE, IS EMPLOYED AS THE ANODE IN ELECTROLYSIS IN A MOLTEN SALT ELECTROLYTE OF

March 19, 1974 T. A. SULLJVAN ETAL PROCESS FOR PRODUCING ALUMINUM ANDSILICON FROM ALUMINUM SILICON ALLOYS Filed Feb.

United States Patent() U.S. Cl. 204-67 10 Claims ABSTRACT OF THEDISCLOSURE Aluminum-silicon alloy produced, for example, by smeltingaluminum-silicate ore, is employed as the anode in electrolysis in amolten salt electrolyte of NaCl-KCl-AlCl3 or NaCl- KCl-AlF3. Moltenaluminum is produced at the cathode. Anode residue is acid leached toproduce metallurgical grade silicon.

This invention relates to treating aluminum-silicon alloys.

Many aluminum-containing domestic ores are unsuitable Yfor use inpresent processes for the production of aluminum. However, research intomethods for the utilizaiton of such ores has shown that many of them canbe utilized for the preparation of aluminum-silicon alloys. Numerousstudies have been made into methods of preparing aluminum-silicon alloysfrom domestic clays, bauxites, and other aluminum ores.

The possibility of extracting aluminum from aluminumsilicon alloys hasbeen investigated using several processes. One approach has been toextract aluminum by dissolution with another metal such as zinc,magnesium, or mercury. The aluminum is then recovered from the solutemetal by distillation or other means. Technical ditlculties in both thesolution of the aluminum and in the distillation step have hindered theutilization of these processes.

A process for treating aluminum-silicon alloy with aluminum monochlorideto extract the aluminum has also been explored. While the technicalfeasibility of the method was demonstrated, other diiculties haveprevented the commercial utilization of this process.

Several processes for the use of low-temperature electrolytes (90-600C.) for the extraction of aluminum from crude aluminum have beenpatented. All of these.

processes rely on the deposition of solid aluminum at the cathode. Thenecessity for frequent removal of the dendritic deposits on the cathodesand consolidation of the aluminum without oxidation have been the chiefobstacles to utilization of these low-temperature processes.

We have now developed a process for electrowinning aluminum fromaluminum-silicon alloys by molten salt electrolysis in NaCl-KCl-AlCl3 orNaCl-KCl-AlF3 electrolytes. Additionally, a metallurgical grade siliconproduct can be prepared by acid-leaching the residue of the electrolysisstep from the anodes of aluminum-silicon alloys that contained onlyaluminum and silicon as the major elements.

By combining such molten salt electrolysis with a smelting step, aprocess is provided for extracting aluminum from aluminum silicate orematerials and clays.

The phrase metallurgical grade silicon as used throughout thespecications and claims means a purity of at least 98.5 percent silicon.

It is therefore an object of the present invention to separate aluminumfrom aluminum-silicon alloys.

Another objective is to recover metallurgical grade silicon.

3,798,140 Patented Mar. 19, 1974 A still .further object is to recoversaid aluminum in a molten state.

In the practice of the present invention the aluminum silicate sourcematerial such as clay or otfgrade bauxite is smelted in the prior artmanner so as to produce Al-'Si alloys. U.S. Bureau of Mines Report ofInvestigation No. 5575 (1960) describes such a smelting procedure.Carbothermic smelting of such materials is described in (l) The ChemicalBackground of Aluminum Industry, Royal Institute of Chemistry, London,1955, pp 72-82, and (2) Carbothermic Smelting of Aluminum, AluminumCompany of America, Alcoa Research Laboratories, New Kensington, Pa.,1964, pp. 42.45.

Thereafter, the aluminum-silicon alloy is employed as the anode inmolten salt electrolysis. The theory of operation of the electrolyticprocess is the selective electrolytic oxidation of aluminum from analuminum-silicon alloy anode in a molten salt electrolyte and thesimultaneous reduction and recovery of aluminum at the cathode. Silicon,undissolved aluminum, and other impurities remain at the anode fromwhich the silicon can be recovered as metallurgical-grade silicon.Molten electrolyte used in the process is used at an electrolytetemperature range of 670 C. to l,000 C. so that the aluminum recoveredas metal is in a liquid form to facilitate its removal from the systemand reduce electrolyte dragout losses. The reactions taking. placeduring electrolysis in the molten electrolyte may be expressed:

At the anode:

Where x and y represent the proportions of A1 and Si in various alloycompositions. Thus, aluminum-silicon alloys are broken down into theirelements by the electrolytic oxidation of the aluminum, leavingelemental silicon and the reduction of the extracted aluminum ion toaluminum metal.

Exemplary apparatus used in the extraction of aluminum fromaluminum-silicon alloys is shown in the figure. Reference numeral 1designates a nickel crucible with a flanged top and cooling gland 2. Agraphite crucible 3 inserted in the nickel crucible 1 is used to containthe molten electrolyte 4. A perforated graphite anode crucible 5 is usedto contain Al-Si alloy anode 6. A graphite support ring 7 resting on topof the anode crucible 5 supports an alumina crucible 8 in which theextracted aluminum 9 is collected and also supports a perforatedgraphite shield 10 which surrounds the alumina crucible 8 and thecathode 11. The cell lid 12 rests on a rubber gasket 13 and is sealed tothe nickel crucible by means of C-clamps 14. A sliding seal of rubbertubing 15 provides an air-tight seal on the cathode lead 16 and permitsraising and lowering of the cathode. The anode lead 17 is electricallyinsulated from the cell lid. The space over the electrolyte is filledwith an inert gas through the gas ports 18. The cell is heated to theoperating temperature in a resistance heating furnace.

To operate, the cell is trst charged with the salts that compose theelectrolyte. The cell temperature is raised to 700- C. to melt thesalts. Any moisture in the salts is removed by sweeping the atmosphereover the molten salts with a ilow of inert gas (helium, argon, ornitrogen may be used). When the salt is molten, the aluminumsiliconalloy to be used is charged into the anode crucible S and lowered intothe molten salt 4 along with the alumina crucible 8 and shield 10. Thelid 12 of the cell is clamped on the cell, and any air admitted isreplaced by flushing with inert gas by means of ports 18. Direct currentis then applied to the anode and cathode leads 17 and 16, respectively,and electrolysis is started. On electrolysis, the aluminum issolubilized by electrolytic oxidation at the anode and reduced on thesurface of the cathode 1'1. Because the temperature of operation ishigher than the melting point of aluminum, the aluminum depositedon thecathode drips oi and is collected in the alumina crucible 8.Electrolysis is continued until the majority of the aluminum content ofthe aluminum-silicon alloy has been removed. The aluminum is recoveredby opening the cell and removing the alumina crucible 8. The anodecrucible containing the anode residue is also removed and the residuedumped from it.. A new charge of aluminum-silicon alloy is added to theanode crucible 5 and replaced in the cell. The aluminum in the aluminacrucible is poured into a mold and the alumina crucible 8 returned tothe cell. The cell lid is then replaced and the cathode 11 inserted andelectrolysis started again. The aluminum prepared is primary grade (atleast 99% purity) or better aluminum. The anode residue from the anodebasket may be acid leached to remove any residual aluminum, leaving thesilicon as a metallurgical-grade silicon (at least 98.5% purity). 'I'heperforated graphite screen is used to prevent any ine silicon liberatedat the anode from oating into the cathode compartment and contaminatingthe aluminum metal.

In the following examples, the specilic apparatus and proceduresdescribed above were employed to test the process of the presentinvention of various aluminumsilicon alloys.

EXAMPLE l A commercial 50-50 aluminum-silicon alloy (50.0 percent A1,45.8 percent Si, and 0.3 percent iron) was processed using anNaCl-KCl-AlC13 electrolyte at 750 C. The electrolytewas an equimolarmixture of NaCl and KCl, to which 10% A1Cl3 was added to provide thealuminum carrier ion. Aluminum of 99.98 percent purity was prepared. Arecovery of 82 percent of the aluminum and a cathode current eiiicieucyof 98 percent were achieved.

EXAMPLE 2 A commercial 65-35 aluminum-alloy (65.0 percent Al, 34.2percent Si, and 0.13 percent Fe) was processed using an NaCl- KCl-AlCl3electrolyte of the same cornposition as Example 1 at 750 C. Aluminum of99.99 percent purity was prepared. A recovery of 88 percent of thealuminum and a cathode current eflciency of 91` percent were achieved.

EXAMPLE 3 An aluminum-silicon-iron alloy (19.6 percent Al, 55.3 percentSi, 19.7 percent Fe, and 0.2 percent Ti) prepared by smelting ofaluminum silicate was processed using an NaCl-KCl-AlCl3 electrolyte(same as previous examples) at 750 C. Aluminum of 99.70 percent puritywas prepared. A recovery of 94 percent of the aluminum and a cathodecurrent efliciency of 89 percent were achieved.

EXAMPLE 4 A crude aluminum-silicon alloy (60.0 percent A1, 26.7 percentSi, 6.1 percent Fe, 4.2 percent Ti, and 0.5 percent C) was processedusing an NaCl-KCl--AlCl3 electrolyte (same as previous examples) at 750C. Aluminum of 99.99 percent purity was prepared. A recovery of 82percent of the aluminum and a cathode current etliciency of 90 percentwere achieved.

EXAMPLE 5 A reiined aluminum-silicon alloy (65.1 percent Al, 23.5percent Si, 9.5 percent Fe, 1.3 percent Ti and 0.1 percent '4 C wasprocessed using an NaCl-KCl-AlCl3 electrolyte (same as previousexamples) at 750 C. Aluminum of 99.99 percent purity was prepared. Arecovery of 81 percent of the aluminum and a cathode current eiiiciencyof percent were achieved.

EXAMPLE 6 An aluminum-silicon casting alloy (87.0 percent Al, 11.9percent Si, 1.5 percent Fe and 0.2 percent Ti) was processed using anNaCl-KCl-AlCl3 electrolyte (same as previous examples) at 750 C.Aluminum of 99.99 percent purity was prepared. A recovery of 89 percentof the aluminum and a cathode current eciency of 81 percent wereachieved.

EXAMPLE 7 A commercial 50-50 aluminum-silicon alloy (same as Example 1)was processed using an NaCl-KCl-AIF3 electrolyte at 750 C. Theelectrolyte was composed of equimolar mixture of NaCl and KCl, with 7%added AlF3. Aluminum of 99.98 percent purity was prepared. A recovery of97 percent of the aluminum and a cathode curent eiliciency of 96 percentwere achieved.

EXAMPLE 8 A commercial 65-35 aluminum silicon (same as Example 2) wasprocessed using an NaC1-KCl--AlF3 electrolyte (same as Example 7) at 750C. Aluminum of 99.95 percent purity was prepared. A recovery of 93percent of the aluminum and a cathode eiciency of 93 percent wereachieved.

IEXAMPLE 9 An aluminum-silicon-iron alloy (same as Example 3) wasprocessed using a NaCl-KCl-AlFs electrolyte (same as Example 7) at 750C. Aluminum of 99.83 percent purity was prepared. A recovery of 94% ofthe aluminum and a cathode current eiciency of 92 percent were achieved.

EXAMPLE 10 The anode residue from a test using a commercial- 50-50aluminum silicon alloy (same as Example 1) was leached using hot dilutehydrochloric acid. After ltering, washing, and drying, the undissolvedsilicon product contained only `0.33 percent aluminum and 0.23 percentiron and met metallurgical grade silicon specifications (98.5% silicon).

EXAMPLE 1l The anode residue from a test using a commercial 65-35percent aluminum silicon alloy (same as Example 2) was leached using hotdilute hydrochloric acid. After iiltering, washing, and drying theundissolved silicon product contained only 0.25 percent aluminum and0.11 percent iron and met metallurgical grade silicon speciiications.

The preparation of aluminum from aluminum silicon alloys using a widevariety of alloy compositions has been accomplished. The aluminumcontent of the alloy has ranged from 19.6 to 87 percent aluminum. Thesilicon content of the alloys has ranged from 11.9 to 55.3 percentsilicon. In addition, the alloys contained iron ranging from 0.1 to 19.7percent iron. Successful extraction of aluminum from all these alloysWas demonstrated.

The electrolyte composition of equimolar quantities of NaCl and KCl with5 to l5 percent A1C13 is most suitable for the all-chloride electrolyte.The electrolyte composition of equimolar quantities of NaCl and KCl with3 to 10 percent AlF3 is most suitable for the chloride-iluorideelectrolyte.

Electrolysis in the chloride electrolyte is most favorable at aneffective voltage of 1.2 volts and to 150 amps/ ft?. Electrolysis in thechloride-fluoride is most favorable at an effective voltage of 2.0 voltsand -200 amps/ ft?.

The most favorable temperature range for the electrolytic process isfrom 700 to 800 C.

In the leaching of the anode residue, suitable acids such as HC1 orH280.,g can be employed. Acid concentrations are 5 to 30 percent.

What is claimed is:

1. A process for removing aluminum from an aluminum-silicon alloycomprising electrowinning said aluminum by molten salt electrolysis at atemperature of about 670 C.1G00 C. with an electrolyte selected from thegroup consisting of NaCl- KCl--AlCla and Nac1-Kc1--A1F3 2. The processof claim 1 wherein the anode residue of said electrolysis is leached4with an inorganic acid to remove impurities and leave behindmetallurgical grade silicon.

3. The process of claim 1 wherein said temperature is about 700-800 C.

4. The process of claim 1 wherein said electrolyte is NaC1-KCl-AlCl3containing equimolar quantities of NaCl and KCl with 5 to 15% A1Cl3.

5. The process of claim 1 wherein said electrolyte is NaC1-KCl-A1F3containing equimolar quantities of NaCl and KCl with 3 to 10% A1F3.

6. The process of claim 1 wherein said aluminumsilicon alloy is producedby smelting an aluminum-silicate ore material.

7. The process of claim 2 wherein said temperature is vabout 7Go-800 c.

References Cited UNITED STATES PATENTS 2,598,777 6/1952 Frary 204-67 X2,937,929 5/ 1960 Voos 423--348 3,148,131 9/1964 Coursier et al. 423-348X JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant ExaminerU.S. Cl. X.R. 423-349

